Electrospinning of polyamide nanofibers

ABSTRACT

The invention relates to a process for the preparation of polyamide nanofibers by electrospinning, wherein the process is a multi-nozzle electrospinning process with the use of a multi-nozzle device or a nozzle-free electrospinning with the use of nozzle free device, comprising steps wherein a high voltage is applied, a polymer solution comprising a polymer and a solvent is fed to the multi-nozzle device or the nozzle free device and transformed under the influence of the high voltage into charged jet streams the jet streams are deposited on a substrate or taken up by a collector, and the polymer in the jet streams solidifies thereby forming nanofibres, and wherein the polymer comprises a semi-crystalline polyamide having a C/N ratio of at most 5.5 and a weight average molecular weight (Mw) of at most 35,000. The invention also relates polyamide nanofibers made by the electrospinning process, as well as to products made thereof and use thereof.

The invention relates to a process for the preparation of polyamide nanofibres by electrospinning, more particular to large scale electrospinning of polyamide nanofibres and production of nanofiber membranes. The invention also relates to nanofiber membrane constructions with a non-woven fiber web of such polyamide nanofibers, and products made thereof and use thereof.

Polymeric nanofibers, also known as fine polymer fibers, can be produced in the form of non-woven fiber webs which are useful in many applications, such as filters. Polymeric nanofibers are also used in a broad range of other fields. These fields include, inter alia, tissue engineering, specialty filters, reinforcements, protective clothing, catalyst supports, and various coatings.

One technique conventionally used to prepare fine polymer fibers is the method of electro-spinning. The nanofibers can be produced from different polymers, and selection thereof will depend on the requirements on stability against heat, humidity, reactive materials, mechanical stress etcetera, in relation to the intended use. Polymers used for preparing nanofibers include, amongst others, polyamides.

Nanofibers made from polyamides are describes for example in US2004/0060268. The polyamides used in the said patent application are nylon 6,66,610, nylon66 and copolyamides of nylon 46 and nylon 66. Although US2004/0060268 mentions that the polyamide nanofibers described therein are produced by electrospinning, not many details are revealed about said process.

Electrospun fibers made from Polyamide 11 and polyamide 12 are described in US20090042029. The polyamides were spun from solutions comprising formic acid and dichloromethane. Contrations of the spinning solutions were low (3-5 wt %), while the nanofibres where often ribbon shaped and characterized with a relative broad fibre diameter distribution.

Large scale electrospinning is typically done via multi-nozzle electrospinning with the use of multi-nozzle devices, for example as described in WO2005/073441, hereby incorporated by reference, and via nozzle-free electrospinning with the use of nozzle free devices, for example using a Nanospider™ apparatus, bubble-spinning or the like; or via electroblowing, for example as described in WO03/080905, hereby incorporated by reference. Many scientific studies however, are done with a single nozzle set-up, such as with the use of a syringe with a needle.

The process of electrospinning in general and of polyamide nanofibers in particular is faced with several problems, in particular when performed on industrial scale using spinnerets with multiple nozzles. These problems are well known, e.g. from WO-2005/033381-A2, WO-2005/073441-A1 and US20090123591.

WO-2005/033381-A2 describes the electrospinning in the following details. When an external electrostatic field is applied to a conducting fluid (e.g., a charged semi-dilute polymer solution or a charged polymer melt), a suspended conical droplet is formed, whereby the surface tension of the droplet is in equilibrium with the electric field. Electro-spinning occurs when the electrostatic field is strong enough to overcome the surface tension of the liquid. The liquid droplet then becomes unstable and a tiny jet is ejected from the surface of a spinneret tip. As it reaches a grounded target, the jet stream can be collected as an interconnected web of fine sub-micron size fibers. The resulting films from these non-woven nanoscale fibers (nanofibers) have very large surface area to volume ratios.

As described in WO-2005/033381-A2, it is important to realize that in the process for manufacturing electrospun fibers major technical problems have to be dealt with. The major technical effect is the speed of fabrication. For example, if one considers a polymer melt being spun from the spinneret with nozzles having a diameter of 700 μm, and the final filament is formed with a diameter of 250 nm, the draw ratio will then be about 3×106. As the typical throughput of the extrudate from a single spinneret is about 16 mg/min (or 1 g/hr), the final filament speed will be about 136 m/s, as compared to the highest speed (10,000 m/min or 167 m/s) attainable by the high speed melt spinning process. Thus the throughput of the spinneret in conventional electrospinning is about 1000 times lower than in the commercial high speed melt-spinning process.

Another major technical problem for mass production of electrospun fibers is the assembly of spinnerets during electrospinning. A straightforward multi-jet arrangement as in high speed melt spinning cannot be used because adjacent electrical field often interfere with one another.

The device described in WO-2005/073441-A1 comprises a nozzle block comprising multiple nozzles, a collector for collecting fibres being spun from the nozzle block and a voltage generator for applying a voltage to the nozzle block and the collector. The polymer used in WO2005/073441 is a nylon 6 with a relative viscosity of 3.2 (determined in a 96% sulfuric acid solution), which corresponds with an Mw of 48,000 g/mol. The polymer is used in for making spinning liquids, e.g. with a viscosity of 1050 cPs at 20% solids. According to WO2005/073441, electrospinning is generally carried out at a very low throughput rate of 10″² to 10^(˜3) g/min per hole. For that reason electrospinning done at one hole is not suited for mass production needed for commercialization purposes. Large scale electrospinning devices used for mass production needed in commercialization, comprise a plurality of nozzles which should be arranged in a narrow space. However, in the conventional electrospinning devices, it is impossible to arrange a limited number of nozzles in a predetermined space, thus making mass production needed for commercialization difficult. Further, the conventional horizontal electrospinning devices has another problem that there occurs a phenomenon (hereinafter, referred to as ‘droplet’) that a polymer liquid aggregate not spun through the nozzles is adhered to a collector plate, thereby deteriorating the quality of the product. To solve this problem WO2005/073441 proposes a so-called bottom-up electrospinning device, wherein the outlets of the nozzles are installed on the nozzle block in upward direction and the collector is located on top of the nozzle block.

The major technical barriers for manufacturing nanofibers by electro-spinning according to US20090123591, a divisional of U.S. patent application Ser. No. 10/936,568, are the low speed of fabrication and the limitation of the process to polymer solutions. The related issues have been summarized in US20090123591 as follows:

-   1. The first barrier involves electrical field interferences between     adjacent electrodes (or spinning jets), which limit the minimum     separation distance between the electrodes or the maximum density of     spinnerets that can be constructed in the multiple jet     electro-spinning die block. -   2. The second barrier is related to the low throughput of the     individual spinneret. In other words, as the fiber size becomes very     small, the yield of the electro-spinning process becomes very low. -   3. The third barrier is limited by the capability for continuous     operation over extended periods of time and automatic cleaning of     multiple spinnerets with minimal labor involvement. -   4. The last barrier of electro-spinning is due to the limitation of     solution processing, where the use of solvent severely hinders the     industrial applicability of the technique.

US20090123591 proposes a special spinneret format as a solution to overcome the technical hurdles (2)-(4) of the conventional electro-spinning technology, as well as to affect (1) the flow of fluid jet streams by gas-blowing. However, not all electrospinning processes can be modified with gas blowing, and conventional set-ups do not comprise gas-blowing.

US20090123591 cites another patent application, WO 03/080905, which addresses the same problems and which proposes a high-throughput production method based partially on electro-spinning: A manufacturing device and the method of preparing for the nanofibers by electro-blown spinning process. According to US20090123591, there are several drawbacks in this disclosed technology, which include the following: it does not fully utilize the electrical field to achieve a sufficiently large spin-draw ratio during blowing, thus, they cannot produce smaller size diameter fibers (e.g., fibers of less than 300 nm in diameter). It cannot sustain a long-term operation capability (e.g., >5 days) because the unavoidable polymer deposits (accumulations) on the spinneret will pose a major problem for sustained operation.

Publications on electrospun fibers often rely on monofilament experiments, but ignore the problems faced with during electrospinning of multifilaments in mass production.

For example, S. S. Ojha et al, in J. Appl. Polymer Sc., Vol. 108, 308-319 (2008) describe the morphology of electrospun fibres as a function of molecular weight and processing parameters. The polymer solutions were prepared from nylon-6 polymers with different Mw, in formic acid. The solutions were applied from a syringe with a capillary tip. It was shown that low Mw nylon 6 (Mw 30,000) showed severe bead formation. This bead formation decreased when increasing concentration as well as when going to Mw 50,000, and even more so when going to Mw 63,000.

Patent application FR-2911151-A1 also describes the production of polyamide nano-fibres with the use of an apparatus equipped with a syringe and a needle. Solutions of polyamide 6 and polyamide 6,6 with Mw of 10,000-50,000 in formic acid are described. Nanofibres with fine diameters in the range of 40-350 nm, but with a wide distribution, are obtained.

C. Huang et al, in Nanotechnology 17 (2006) 1558-1563, describe electrospun polymer nanofibres with small diameters prepared from polyamide 4.6 in formic acid. The polyamide 4.6 was a grade from Aldrich, which was determined to have an Mw of 45.000. A small amount of pyridine was added to the electrospinning solution to avoid the formation of beaded nanofibres in the course of spinning at low concentrations. At such low concentrations fibres with small diameters were obtained. At higher concentrations, above 12% solids, the diameter of the nano-fibres appeared to increase drastically, almost in an exponential manner.

Another requirement for large scale commercial electrospinning is the long time stability of the viscosity of the polymer solution. In general the polymer used for electrospinning is dissolved and the solution is stored into relatively large containers or tanks and takes up to several weeks prior to be completely consumed in the electrospinning process. As the viscosity of the polymer solution is strongly affecting the resulting fiber diameter of the nanofibers and distribution thereof, it is important to have a stable polymer-solvent system.

Thus there is a clear need for an improved electrospinning process for the preparation of nanofibers, more particular for polyamide nanofibers, that can be used for large scale production.

The aim of the present invention is to provide an electrospinning process that does not have all the above disadvantages, i.e. or at least so in reduced extent, which is suitable for making nanofibres with small diameters, narrow fibre distribution, and limited bead formation, meanwhile allowing for high throughput, and can be performed on large scale.

This aim has been achieved with the electrospinning process according to the invention, wherein the process is a multi-nozzle electrospinning process with the use of a multi-nozzle device or a nozzle-free electrospinning with the use of nozzle free device, comprising steps wherein

-   -   a high voltage is applied     -   a polymer solution comprising a polymer and a solvent is fed to         the multi-nozzle device or the nozzle free device and         transformed under the influence of the high voltage into charged         jet streams     -   the jet streams are deposited on a substrate or taken up by a         collector, and     -   the polymer in the jet streams solidifies thereby forming         nanofibres and wherein the polymer comprises a semi-crystalline         polyamide having a C/N ratio of at most 5.5 and a weight average         molecular weight (Mw) of at most 35,000.

The C/N ratio is herein understood to be ratio between the number of carbon atoms (C) in the polyamide and the number of nitrogen atoms (N) in the polyamide.

The process according to the invention is an electrospinning process, wherein multiple nanofibres are formed simultaneously, and that can be performed on large scale.

In such processes Taylors cones are formed from the solution either from the nozzles of from a free standing liquid when applying a high voltage. To create such Taylor cones the voltage typically has to be at least 2.5 kV. The voltage may be as high as 50 kV or 60 kV or even higher, e.g. 65 kV. Suitably the voltage is at least 10 kV, preferably at least 20 kV and more particular at least 30 kV.

The process can be done via multi-nozzle electrospinning with the use of multi-nozzle devices, typically a spinneret with a series of nozzles, and via nozzle-free electrospinning with the use of nozzle free devices, for example using a Nanospider™ apparatus or bubble-spinning. Multi nozzle spinning may optionally be combined with a forced air flow around the nozzles, as in electro-blowing.

Such a large scale multi-nozzle electrospinning process comprises steps wherein

-   -   the high voltage is applied between a spinneret and a collector,         or between a separate electrode and a collector, the spinneret         comprising a series of spinning nozzles,     -   a stream of a polymer solution comprising a polymer and a         solvent is fed to the spinneret,     -   the polymeric solution exits from the spinneret through the         spinning nozzles and transforms under the influence of the high         voltage into charged jet streams,     -   the jet streams are deposited on or taken up by the collector or         a substrate,     -   the polymer in the jet streams solidifies prior to or while         being deposited on or taken up by the collector or a substrate,         thereby forming nanofibres.

In a nozzle free process, no spinneret with nozzles is present, and another device is present to which the solution is fed, and from which the yet streams are formed. For example, the solution is taken up by a rotating electrode, as in a nanospider apparatus, or bubbles are being formed from the solution by purging gas through the solution (as in bubble spinning). In a nozzle free process, the solution fed by such means, produces a series of Taylor cones under the influence of the high voltage. From these Taylor cones charged jet streams are being formed above a critical voltage, which end up in the nanofibres as described above for the multi-nozzle process.

In a first embodiment of the process according to the invention, the polymer in the polymer solution comprises a semi-crystalline polyamide having a C/N ratio of at most 5.5 and a weight average molecular weight (Mw) of at most 35,000.

The effect of the process according to the invention wherein the said semi-crystalline polyamide with the C/N ratio of at most 5.5 and the Mw of at most 35,000 is used, are multiple. The variation in fiber thickness within the combination of multiple fibers produced is reduced (or by addition of high Mw branched polyamide. Polymer solutions with higher polyamide concentrations, e.g. above 15 wt %, and even above 20 wt. % can be used resulting in higher throughput. Process conditions can be selected purposely aiming at either thin or thick fibers, while at the same time maximizing the production output rate and being able to control the average fiber thickness or fiber diameter. The fiber thickness of the nanofibers produced with the process according to the invention can be varied over a wide range: i.e. with the process, process conditions can be chosen such that relatively thin fibers (e.g. by lowering solution viscosity, lowering molecular weight of polymer) as well as relatively thick fibers (e.g. by increasing solution viscosity, increasing ethanol content in formic acid/ethanol mixtures) can be produced.

The process results in less nozzle blocking at comparable solvent composition and concentrations for corresponding and other polyamides with higher Mw. The process allows greener solvent compositions, in particular solvent compositions with higher water content. In contrast to said standard semi-crystalline polyamide with a C/N ratio of more than 6, the polyamides used in the process according to the invention allow to be processed at higher water contents in formic acid/water mixtures.

The process allows polyamide polymers with high melting temperatures that otherwise are not processable or degrade to fast during melt processing, to be processed into nanofibres.

These results are surprising in view of the facts that with other polyamides, concentrations higher than 20% for nylon 6 and 10% for nylon 6,66,610 cannot be achieved in large scale multi-nozzle spinnerets, and variation in the solution viscosity of these polymers does not result in a wide variation of the fiber thickness. With nylon 6, fibres with a diameter around 100 nm are obtained, while with nylon 6,66,610 fibres with diameter in the range of 200-300 nm are obtained. Reducing the Mw of nylon 6 does not result in a smooth electrospinning process and does not lead to the production of decent nanofibers.

In a second embodiment, the polymer comprises a first polyamide having a C/N ratio of at least 6 and an Mw of at most 35,000 and a second polyamide being a high Mw linear polyamide.

The second polyamide can be a high Mw linear polyamide, optionally anionically polymerized, or a high Mw branched polyamide, preferably present in an amount between 0-10 wt. %, more preferably 0-5 wt. %, relative to the total weight of polymer. Also preferably the Mw of the first polyamide is in the range of 5,000-25,000.

The process according to the second embodiment results in a stable process and a fibrous product with good quality, small fiber diameter distribution and very low amount of bead formation. This in contrast to a corresponding process including the use of a solution comprising only the lower Mw polyamide. Earlier attempts to use polyamides having such a lower Mw for attaining lower viscosity solution and/or higher concentration, typically gave rise to beads, or fibers in combination with beads, in particular with a higher voltage applied for multiple nozzles (for high throughput) and thin fibers and/or low mechanical properties.

With the term semi-crystalline in semi-crystalline polyamide is herein understood a polyamide that in solid form comprises a crystalline phase next to an amorphous phase. The presence of a crystalline phase can be evidenced by various techniques, such as Rontgen diffraction and differential scanning calorimetry (DSC).

The melting temperature of such a partially crystalline polyamide can also be determined by DSC.

The term melting temperature, as used further below, is herein understood to be the temperature, measured according to ASTM D3418-97 by DSC under nitrogen with a heating rate of 10° C./min showing the highest melting rate.

It is further noted that the notation of ranges in the format of a range of x-y, with x the y as the lower and upper limit, these limits are included within the range.

Polyamides that can be used in the process according to the invention are polyamides from linear diamines H2N—(CH2)_(X)—NH2 with X=2, 3, 4, 5, and/or 6 and dicarboxylic acids HO₂C(CH₂)_(Y)CO₂H with Y=0, 1, 2, 3, and/or 4. In case C6 diamines, ie. diamines with 6 C atoms, are combined with C6 dicarboxylic acid, i.e. dicarboxylic acid with 6 C atoms, it is clear that these have to be combined with shorter diamines and/or shorter dicarboxylic acids to arrive at a C/N ratio of 5.5 or lower. Other polyamides that can be used are polyamides from aminoacids H₂N—(CH₂)_(Z)—CO₂H or lactams [NH(CH₂)—_(Z)CO] with Z=1, 2, 3, 4 and/or 5.

Also copolyamides combining the above mentioned monomers with monomeric units comprising 6 or more C-atoms can be used, provided that the overall C/N ratio is at most 5.5. Preferably the copolymers are PA46/6 with more than 70 wt. % PA46 and up to 30 wt. % of PA6-units, and PA46/66 with more than 70 wt. % PA46 and up to 30 mol % PA66 units.

Suitable semi-crystalline polyamides that can be used for the semi-crystalline polyamide having a C/N ratio of at most 5.5 and a weight average molecular weight (Mw) of at most 35,000, include the homopolymers polyamide 46, polyamide 26, polyamide 24, polyamide 4, polyamide 36, and polyamide 56, and copolymers thereof.

Preferably, the C/N ratio is in the range of 4-5.5, more preferably 4.5-5.25.

Also preferably, the semi-crystalline polyamide comprises a homopolymer, more preferably the homopolymer is nylon 46, which has a C/N ratio of 5, and nylon 26, which has a C/N ratio of 4.

Polyamides having a low C/N ratio, such as polyamide 26 and 24 and copolymers thereof, in particular the homopolymers thereof, are generally characterized by a very high melting point, and these polymers typically degrade when processed at high temperature. This degradation prevents these polymers to be processed at high temperature, in particular in melt processing thereof, and in many cases it is also impossible to produce high Mw products thereof. With the present process it is possible to make nanofibres having good properties.

The semi-crystalline polyamide with the C/N ratio of at most 5.5 can be present in an amount varying over a wide range, compared to total amount of the polymer. Suitably, the amount of the semi-crystalline polyamide is at least 50 wt. %, more preferably in the range of 75-100 wt. %, and most preferably 90-100 wt. %, relative to the total amount of polymer present in the nanofibres.

The weight average Mw of the semi-crystalline polyamide may vary over a wide range as long as it is at most 35,000. The weight average molecular weight (Mw) referred to herein is determined by measuring the molecular weight distribution by gel permeation chromatography (GPC), more particular with Size Exclusion Chromatography (SEC) combined with triple detection method. Here for a GPC apparatus is coupled to viscometry, refractive index and light scattering detection (90 degrees). The measurements are performed using hexafluoroisopropanol comprising 0.1 wt % potassium trifluoro acetate, relative to the weight of the hexafluoroisopropanol, as solvent, and employing a Size Exclusion Chromatograph equipped with 3 PFG linear XL silica columns; The weight average molecular weight is calculated from the measured molecular weight distribution using TriSEC 3.0 software of the company Viscotek. The Mw is expressed in g/mol. The triple detection method has the advantage in that this method gives absolute values and does not need an external reference.

Suitably the Mw is at least 1,000, preferably at least 2,000, more preferably in the range of 5,000-30,000, and still more preferably in the range of 10,000-25,000. The advantage of a lower Mw is that even higher concentrations may be used in the process, and/or the solvent may comprise a higher water or alcohol content.

The very low Mw polyamides, i.e. with an Mw in the range of 1,000-5,000, are suitably used in combination with a second polyamide being a high Mw polyamide. The second polyamide can be a high Mw linear polyamide, optionally anionically polymerized, or a high Mw branched polyamide, preferably present in an amount between 0-10 wt. %, more preferably 0-5 wt. %, relative to the total weight of polymer. An advantage of using such low Mw polyamide polymers is increased adhesion between separate nanofibers in the produced nanofiber web and increased productivity.

The polyamide having a C/N ratio of at most 5.5 and an Mw of at most 35,000 may also consist of a mixture of different polyamides. For example, the mixture may consist of a polyamides with very low Mw, e.g in the range of 1,000-5,000, in combination with a polyamide with an Mw in the range between 5,000 and 35,000 or equal to 35,000.

The polymer in the solution may comprise next to the semi-crystalline polyamide another polymer or other polymers. Other polymers that may be present can be any polymer that is soluble or dispersible in the solvent used in the solution. Such a polymer is preferably present in an amount of at most 50 wt. %, more preferably in the range of 0-25% wt, and still more preferably in the range of 0-5 wt %, relative to the total weight of the polymer.

The other polymer may for example be another polyamide, referred to as the second polyamide. Thus the polymer may comprise a mixture of polyamides, comprising the polyamide having a C/N ratio of at most 5.5 and an Mw of at most 35,000 referred to as the first polyamide, and the second polyamide. The second polyamide may be of the same chemical composition as the first polyamide, or be of different chemical composition. Suitably, the second polyamide is a high Mw linear polyamide or a high Mw branched polyamide. The high Mw polyamide may well be an anionically polymerized polyamide. The second polyamide may also well be a polyamide having a C/N ratio of more than 5.5 and/or an Mw of at more than 50,000, preferably more than 70,000, more preferably more than 100,000 g/mole and up to 200,000 g/mol or higher.

Preferably the second polyamide is present in an amount in the range of 0-10 wt. %, preferably 0.5-5 wt. %, relative to the total weight of polymer. The advantage of the second polymer being present in such a limited amount, in particular the high Mw polyamide having an Mw of at more than 50,000 or a branched modification thereof, is that the shear viscosity of the solution can be kept low while still nanofibres with relatively high thickness can be produced.

The solvent used for the polymer solution suitably consists of a solvent mixture. More particular, the solvent mixture comprises a combination of different polar solvents. These polar solvents may be chosen from acids, alcohols, water, optionally combined with less polar solvents such as esters and/or solubility enhancers such as salts.

Suitable solvent mixtures include a mixture comprising (I) water, a water soluble salt, and either methanol, ethanol, glycol and/or glycerin, and optionally comprising either NH3, an aliphatic amine and/or a diamine or a mixture comprising (II) formic acid and/or acetic acid, and at least one liquid selected from the group consisting of water, methanol, ethanol, glycol, glycerin, and methyl formate. Also pure formic acid, or a mixture of formic acid and acid can be used.

The hydrophilic nature of the polyamide used in the process according to the invention, in combination with its low Mw, allow the use of solvent mixtures with a relatively high water and/or alcohol content.

In the solvent mixture comprising water in combination with formic acid, the water is preferably present in an amount of at least 15 wt. %, more preferably in the range of 20-30 wt. %, relative to the total weight of the solvent mixture.

In the solvent mixture comprising water in combination with methanol and salt, the water is preferably present in an amount of at least 30 wt. %, more preferably in the range of 40-60 wt. %, relative to the total weight of the solvent mixture

The advantage of a higher water content is that the process is more environmentally friendly, and lower solvent costs, occurrence of a faster phase separation and earlier solidification of the polymer in the jet stream.

The possibility of higher water content can also be used to enhance the flexibility of the process. The water content may be increased by adding water to the feed stream, i.e. to the solution during the feeding step, prior to entering the spinneret. By doing so, one can use a single storage tank comprising a solution with a high polymer concentration, different concentrations and fiber diameters. By adding the water one can control the concentration of the polymer in the solution and/or the viscosity of the solution and/or the speed of phase separation after leaving the spinneret thereby steering the resulting fiber diameter. The water may be added as such, or preferably diluted with a part of the cosolvent or cosolvents. This can be done in order to prevent too big local differences in solvent composition inducing premature precipitation of the polymer.

The polymer in the solution comprising water may also comprise, next to the solvents and polymers describe above, a water soluble polymer. The water soluble polymer may be extracted with water from the nanofibres produced thereby obtaining microporous nanofibres. Suitably the water soluble polymer is polyvinylpyrrolidone (PVP).

The solution may further comprise one or more additives

Suitable additives include surface tension agents or surfactants (e.g. perfluorinated acridine), crosslinking agents, viscosity modifiers (e.g. hyperbranched polymers such as HYBRANE), electrolytes, antimicrobial additives, adhesion improvers, e.g. maleic acid anhydride grafted rubber or other additives to improve adhesion with a PP or PET substrate, nanoparticles, such as nanotubes or nanoclays, and so on. Suitable electrolytes include water soluble metal salts, such metal alkali metal salts, earth alkali metal salts and zinc salts. Examples of suitable electrolytes are LiCl, HCOOK (potassium formate), CaCl2, ZnCl2, Kl3, NaI₃. Preferably the electrolyte is present in an amount in the range of 0-2 wt. %, relative to the total weight of the solution. The water soluble salt may be extracted with water from the nanofibres produced thereby obtaining microporous nanofibres.

The incorporation of nanoparticles into polymeric nanofibers holds additional appeal because nanoparticles can alter or even enhance the mechanical, electrical, thermal, magnetical, optical and chemical properties of the fibers.

In the electrospinning process the polymer in the jet streams solidifies thereby forming the nanofibres, and the jet streams and/or the nanofibres resulting there from are deposited on or taken up by the collector or a substrate.

Suitably, the nanofibres are taken up by a roll or mandrel by semi-continuous winding. These nanofibers can then also be post-stretched to further improve its properties. Alternatively, the nanofibres can be collected on a substrate or collector plate, thereby forming a non-woven web of nanofibres.

The nanofibres obtained from the process according to the invention, irrespective of being obtained as continuous winded fibers or as non-woven web or in any other shape, may be subjected to one or more further processing steps. The nanofibres may for example be washed, dried, cured, annealed and/or post condensed.

Once the polymer in the jet streams has solidified and nanofibres are formed, the nanofibres are suitably washed with water, since the polymer is non-soluble in water, and subsequently dried. Drying is preferably done at a temperature above 100° C. This temperature may well be as high as 200° C. or even higher.

A curing step is advantagely applied if the polymer solution and the nanofibers made thereof comprise a crosslinking agent or an adhesion improver.

The invention also relates to nanofibers obtainable by the process according to the invention, and specific and preferred embodiments thereof, respectively to nanofibres consisting of a polymer composition comprising a polyamide having a C/N ratio of at most 5.5 and an Mw of at most 35,000.

The polyamide comprised by the composition may be any of the polyamides having a C/N ratio of at most 5.5 and an Mw of at most 35,000, as described above, and also referred to as first polyamide. The same applies for the optional second polyamide, other polymer and additive, that may be comprised by the solution used for the preparation of the nanofibers and will have precipitated together with the first polyamide. Also for these components the examples and embodiments for the polymer type amount etc. apply for the polymer composition.

Preferably, the nanofibers consist of a polymer composition consisting of

-   -   a) 75-100 wt % of the polyamide having a C/N ratio of at most         5.5 and an Mw of at most 35,000     -   b) 0-25 wt. % of a second polymer, being either a polyamide         having a C/N ratio of more than 5.5 and/or an Mw of more than         35,000, and/or another polymer wherein the wt. % of a) and b) is         relative to the total weight of a) and b), and     -   c) 0-25 wt. % of at least one additive, wherein the wt. % of c)         is relative to the total weight of a), b) and c).

More preferably, the first polyamide (a) has a C/N ratio of less than 5.

Also preferably the polyamide in the nanofibers has a melting temperature (Tm) of at least 250° C., preferably at least 270° C., more preferably at least 290° C. and most preferably in the range of 300-350° C.

The invention also relates to nanofibers consisting of a polymer composition consisting of:

-   -   (i) The invention also relates to nanofibers consisting of a         polymer composition consisting of: 75-100 wt % of the polyamide         having a C/N ratio of less than 5 an Mw of more than 35,000,     -   (ii) 0-25 wt. % of a second polymer, being either a polyamide         having a C/N ratio of at least 5 and/or another polymer     -   wherein the wt. % of (i) and (ii) is relative to the total         weight of (i) and (ii), and (iii) 0-25 wt. % of at least one         additive, wherein the wt. % of (iii) is relative to the total         weight of (i), (ii) and (iii).

Such a nanofiber can be produced by the process according to the invention using a polyamide having a C/N ratio of less than 5 an Mw of less than 35,000, followed by a post condensation step or annealing step. An advantage of this process is that the resulting nanofibers have very good mechanical and high temperature properties and such nanofibers could otherwise not be produced.

Preferably the polyamide in the nanofibers according to the invention has a melting temperature of at least 290° C., more preferably in the range of 300-350° C. Such properties can result from the post condensation step or annealing step with nanofibres comprising as the polyamide having a C/N ratio of less than 5, a homopolyamide of for example, PA26, PA4, or PA44, or copolyamides of respectively PA26, PA4, or PA44 comprising limited amounts of comonomers.

The invention also relates to nanofibres obtainable by the second embodiment of the process according to the invention, respectively to nanofibres consisting of a polyamide composition comprising a polyamide having a C/N ratio of more than 5,5 and having an Mw of at most than 35,000, and a linear or branched polyamide having an Mw of more than 35,000.

The nanofibres according to the invention or obtainable by one of the embodiments of the process according to the invention can have a fiber diameter varying over a large range. The fiber diameter may well be in range of 5-500 nm, or even beyond, and preferably is in the range of 50-300 nm, more preferably 100-250 nm. The average diameter described herein is the number average of the diameters, measured by scanning electron microscopy (SEM), over a statistically large enough number of measuring points.

The fiber diameter was determined as follows. Ten (10) SEM images at 5,000× magnification were taken of each nanofiber sample or web layer thereof. The diameter of ten (10) clearly distinguishable nanofibers were measured from each photograph and recorded, resulting in a total of one hundred (100) individual measurements. Defects were not included (i.e. lumps of nanofibers, polymer drops, intersections of nanofibers). The number average fiber diameter for each nanofiber sample was calculated form the one hundred (100) individual measurements.

The nanofibres according to the invention, and various examples and embodiments thereof, may well have the form of continuous multifilament fibers, or of a non-woven web. The non-woven fiber web made of the nanofibers produced by the process according to the invention is very suited for use in membranes. Such membranes can be referred to as nano-fibre membranes or micro-poruous membranes.

The invention also relates to the use of nanofibres according to the invention, and various examples and embodiments thereof, or obtained by an embodiment of the process according to the invention in a micro-porous membrane, as well to a micro-porous membrane made thereof for the use of any one of the following applications: molecular separations and filtration, like gas/gas filtration, hot gas filtration, particle filtration, liquid filtration such as micro filtration, ultra filtration, nano filtration, reverse osmosis; waste water purification, oil and fuel filtration; electrochemical applications, including electro-dialysis, electro-deionization, batteries (e.g. battery seperators) and fuel cells; controlled release applications including pharmaceutical and nutraceutical components; pertraction, pervaporation and contactor applications; immobilization of enzymes, and humidifiers, drug delivery; (industrial) wipes, surgical gowns and drapes, wound dressing, tissue engineering, protective clothing, catalyst supports, and various coatings. The membrane may also be used as reinforcement, for example in combination with films, such as transparent films.

The nanofibres according to the invention, and products made thereof are hydrophilic and therefore very suitable for medical applications; such hydrophilic polymers exhibit very low to no protein agglomeration. The membranes have very high filter efficiency and are advantageously used in combination with polar solvents and water. For wound dressing, the membrane advantageously comprises antimicrobial additives. The antimicrobial additives may be added to the solution prior to the electrospinning process or may be applied, for example, on a porous nanofibre membrane obtainable with the process according to the invention.

The invention is further illustrated with the following Examples and Comparative Experiments.

Methods Molecular Weight Determination by Gel Permeation Chromatography (GPC)

The GPC measurements were performed on a Viscotek GPCmax apparatus (Malvern) coupled to a Viscotek Triple Detector Analyzer (TDA 302) and a Viscotek PDA. Three PFG linear XL 7μ columns (PSS) were applied. The mobile phase was hexafluoroisopropanol comprising 0.1 wt % potassium trifluoroacetate as a modifier. The flow rate was 0.8 ml/min. The SEC apparatus, columns and detectors were operated at 35° C. Data were collected and analyzed using Omnisec 4.6.1 software (Viscotek) based on triple detector (refractive index, viscometry and light scattering) calibration. Samples were dried for 16 hrs under vacuum before dissolution in the solvent (same as mobile phase). Solutions were filtered prior to injection on the GPC apparatus over a 0.45 μm filter (Schleicher & Schüll).

Solution Viscosity

The solution viscosity of the polymer solutions was measured on an Anton Paar Physica MCR 501 rheometer equipped with a C-PTD200-SN80425502 (Peltier) temperature control device suited for cylindrical measurement systems. The measuring system used was a concentric cylinder system CC27 (serial nr 1770, diameter 26.64 mm and concentricity of 6 μm). Disposable cylinders were used, a new one for each individual solution. The solution in the cylinder was covered by a solvent trap containing some water which was attached above the cylinder to prevent/minimize evaporation during the measurement. Typically the sample solution was kept at 25° C., transferred into the disposable cylinder by pouring and next the sample was given 5 minutes of time to reach the measurement temperature of 25° C. The measurement was performed as a steady shear rate sweep from 10 to 1000 s-1 at 25° C. The viscosity value at shear rate 100 s-1 was reported as the solution viscosity in mPa·s. It is noted that in all the measurements on the different solutions the viscosity at shear rates around 100 s-1 was shear rate independent.

Production Rate

The production rate [g/hr] of a continuous nanofibre electrospinning process is defined as the amount of membrane or nanoweb that is produced in one hour. For measuring the production rate, round samples of 47 mm in diameter (so-called disks) are punched out from a layer of continuous spun nanofiber membrane. The weight of five (5) disks is measured and the number average value calculated. From this average value, the production rate is calculated by using the following formula

$\begin{matrix} {P = {\frac{M}{O} \cdot S \cdot W}} & \left( {{Formula}\mspace{14mu} 1} \right) \end{matrix}$

wherein P=production rate [g/hr] M=average weight of deposited material per disk [g] O=surface of a disk [m²] S=line speed [m/hr] W=width of the electrode [m]

Characterization of Nanofibers: Fiber Diameters, Number Average and Distribution and Standard Deviation

To determine the number average diameter of the fibers, ten (10) samples were taken from a nanofiber web layer and scanning electron microscopy (SEM) images at 5,000× magnification were taken for each. The diameter of ten (10) clearly distinguishable nanofibers is measured from each photograph and recorded, resulting in a total of one hundred (100) individual measurements. Defects are not included (i.e. lumps of nanofibers, polymer drops, intersections of nanofibers). The fiber diameter distribution consists of these hundred individual measurements. From these one hundred (100) individual measurements, the number average diameter (d) of the fibers and the standard deviation (S) is calculated.

Materials

PA46 -1/7 Polyamide 46 polymers, all linear, varying in MW from 13,000 g/Mol to 65,000 g/Mol, all prepared internal DSM using standard polymerization methods. PA46-X PA46 sample obtained from Aldrich (product number 44,299-2) PA6-1/2 Polydamide 6 polymers, both linear, Mw 30,000 resp. 41,000 g/Mol, both prepared internal DSM using standard polymerization methods. Formic acid Industrial grade, 95% formic acid, 5% water.

All polymeric materials were characterized by the GPC method described above. PA46-X was a PA46 sample obtained from Aldrich, which was analyzed in more detail with GPC, the results of which are gathered together with those of PA46-1 in Table 1.

TABLE 1 GPC data for PA46-X and PA46-1. PA46-X PA-46-1 Mn [kg/mol] 23 14 Mw [kg/mol] 44 28 Mz [kg/mol] 78 45 Mw/Mn [—] 1.9 1.95 Mz/Mw [—] 1.7 1.6

It appeared that the Aldrich product has a much higher Mw than PA 46-1 and in fact falls outside the range of the present invention. It was further noted that both products showed a Mark-Houwink plot indicative for linear polymer

Preparation of Polymer Solutions for Electrospinning

An amount of polymer powder or granules in the range of 10-30 g was weighted and added to 200 ml of 95% formic acid (Proanalyse grade from Merck). The solution was stirred using a magnetic stirrer for 12 hours at room temperature (25° C.) in a closed flask. Afterwards the solution viscosity at a shear rate of 100 s⁻¹ at 25° C. was measured with the method described above.

Solutions with different concentrations were prepared using different amounts of polymer. The resulting viscosity data were put in a concentration/viscosity plot, which was used to specify the amount of material needed to prepare solutions with a preselected viscosity. Solutions with a viscosity of about for the 600 mPa·s respectively 1000 mPa·s for different polymers and used for the further experiments were prepared in the same way as described above starting from the specified amounts derived from the plots. After the preparation of the solutions, the viscosity thereof was measured. Relevant data for these solutions have been included in Table 2 and 3.

Electrospinning of Nanofibers on a Nozzle Free Electrospinning Apparatus

Nanofibers were spun using a NS Lab 500 from Elmarco s.r.o., Czech Republic, comprising a device chamber, a solution reservoir, a rotating 4-wire cylindrical electrode of 18 cm a top electrode and an air circulation system. The electrode entrians part of the solution on its wires when rotating through the solution reservoir.

As substrate a standard household aluminum foil of about 0.01 mm thickness was used. For all experiments a 4-wire 18 cm wide electrode has been used. The spinning distance and the applied voltage were fixed for all experiments, being respectively 10 cm, 60 kV. Relative humidity in the device chamber was set and controlled at a preset level (45, 38 and 27% RH respectively) and was measured continuously using a standard relative humidity device during each spinning experiment. In order to obtain dry air conditions silica gel particles were placed in the air suction zone under the device. When more humid conditions were required either saturated salt solutions or hot water reservoirs were placed inside the device chamber.

The polymer solution used was supplied with a temperature of 25° C. or 29° C. and kept at that temperature using the air condition system in the device chamber. During the experiments the temperature was monitored and reported. A substrate speed of 0.13 m/min and a rotational frequency of 50 Hz for the lower cylindrical electrode were applied. Each experiment was run for at least 3 minutes and the resulting substrate coated with a deposited layer of nanofibres was collected.

The deposited layer of nanofibres was investigated on production rate, basis weight of the nanoweb, number average diameter of the fibers and fiber diameter distribution.

The results for different polymer solutions at different levels of relative humidity are collected in the tables further below.

TABLE 2 Data and test results for nozzle free electrospinning of Examples I-V and Comparative Experiments A-D. Av. Fiber Mw Conc. Visc T Prod. rate St. diameter St. [kg/mol] [wt. %] [mPa · s] (° C.) [g/hr] dev [nm] Dev Part A, 25° C., 45% RH EX-I PA46-1 25 18.3 596 24.8 0.564 0.008 87 28 CE-A PA46-3 52 12.7 595 24.6 0.352 0.054 130 53 CE-B PA6-1 30 16.5 549 24.6 0.362 0.006 110 39 Part B, 25° C., 27% RH EX-II PA46-2 34 14.8 606 24.6 0.741 0.008 155 29 CE-C PA46-3 52 12.7 595 24.3 0.647 0.010 156 30 CE-D PA6-1 30 16.5 549 24.9 0.537 0.025 93 52 Part C, 29° C., 27% RH EX-III PA46-2 34 14.8 606 29.3 0.89 0.010 156 30 EX-IV PA46-4 19 22 618 29.0 1.18 0.008 127.8 38.2 EX-V PA46-5 13 27 730 29.0 1.697 0.009 190 42

The first part of the data set (Part A) shows a significantly higher production rate obtained with the lower Mw PA46 of Example-I. Much lower production rates are obtained with higher Mw PA46 of Comparative Experiment A, and also with low Mw PA6 of Comparative Experiment B. This lower production rate cannot be simply explained with the lower concentration, as can be seen from the data for PA6 of Comparative Experiment-B, which has only a slightly lower concentration compared to Example-I, but much higher than that of Comparative Experiment-A. Moreover, the fibre diameter for the high Mw PA46 is significantly higher, already at that very low concentration.

The second part of the data set (Part B) shows that at low relative humidity the production rate increases significantly. Production rate increases going from Comparative Experiment-C to Example II, i.e. by lowering the Mw of the polyamide 46. The lowest production rate is obtained for Comparative Experiment-D, i.e. PA-6 with the lowest Mw and highest concentration of all 3 experiments.

The last part of the data set (Part C) shows if the temperature during processing is increased that the production rate is even further increased, as is seen for Example III in comparison with Example II, without hardly affecting the diameter of the fibers and distribution thereof. Furthermore, the production rate can be further increased by further lowering the Mw of PA46, without causing any significant bead formation. This is in contrast with the two comparative experiments B and D which both showed some bead formation.

It was further noted that all examples with PA46 did show very limited variation in the production rate and relative narrow fiber distribution, as can be seen from the low values for the standard deviations, except that Example IV and Example with the PA46 with the lowest Mw's the fibre distribution is slightly broader. These polymers gave thin fibres and the highest production rates, while still no bead formation occurred. These results were achieved with high concentration solutions without the use of other measures, such as the addition of pyridine.

Multi-Nozzle Electrospinning

A multi-nozzle electrospinning set-up was used similar to the one described in WO2005/073441 A1. The spinning distance and the applied voltage were fixed for all experiments, being respectively 12 cm and 32 kV. A substrate speed of 0.7 m/min was chosen. The air temperature and humidity were controlled at 25° C. and 40% RH. For the experiments solutions were prepared from PA46 polymers with different number average molecular weight in formic acid/water 85/15 w/w as solvent. For the an electrospinning process polymer solutions with a viscosity of about 1000 mPa·s were used. Data and results have been collected in Table 3.

TABLE 3 Data and results of the Multi-nozzle electrospinning for Examples VI-VII and Comparative Experiments E-F at 25° C. and 40% RH. Mw Conc. Visc Av. FD Polymer [kg/mol] [wt. %] [mPa · s] [nm] EX-VI PA46-1 25 22.5 1000 137 EX-VII PA46-6 31 20 995 150 CE-E PA46-7 46 16 1041 167 CE-F PA46-3 52 15 975 168

The polymers in the Examples VI and VII according to the invention allowed for a much higher concentration in combination with retention of a low viscosity, and higher throughputs could be achieved, while the electrospinning process ran more smoothly, compared to Comparative Experiments E and F. The results also show that high concentrations can be used and still nano-fibers with small diameters are obtained.

Solution Stability

Specified amount of polymer powder or granules were added to 200 ml of different solvent mixtures and systems. Three different pure solvent components were used in this study, namely 99.9% formic acid, ethanol and water. The composition of the different solvent blends are shown in table 3. The solution was stirred using a magnetic stirrer for 12 hours at room temperature in a closed flask. After these 12 hours the viscosity of the solution was measured on an Anton Paar Physica MCR501 rheometer with the method described above. The flasks were stored at room temperature conditions and the viscosity of the solutions were measured at several moments over a period of 8 weeks. The test results after 4 weeks and 8 weeks are compared with the initial viscosities and the reduction, expressed in percentage of the initial viscosity, are reported in Table 4.

TABLE 4 Data—viscosity reduction of polymer solutions (20 wt. % in 85/15 formic acid/water) Mw viscosity reduction [%] EX/CE Polymer [kg/mol] 28 days 56 days EX-VIII PA46-1 25 15.9 29.2 EX-IX PA46-2 34 23.5 41.9 CE-G PA46-3 52 31.8 69.6

The data in the table show that the solutions with the lower Mw polymer, i.e. Examples VIII and IX according to the invention, have a much better stability in viscosity than the solutions with the high Mw polymer, illustrating that process according to this invention can be run with stand times that are longer than for other polymers. 

1. Process for the preparation of polyamide nanofibers by electrospinning, wherein the process is a multi-nozzle electrospinning process with the use of a multi-nozzle device or a nozzle-free electrospinning with the use of nozzle free device, comprising steps wherein a high voltage is applied a polymer solution comprising a polymer and a solvent is fed to the multi-nozzle device or the nozzle free device and transformed under the influence of the high voltage into charged jet streams the jet streams are deposited on a substrate or taken up by a collector and the polymer in the jet streams solidifies thereby forming nanofibres and wherein the polymer comprises a semi-crystalline polyamide having a C/N ratio of at most 5.5 and a weight average molecular weight (Mw) of at most 35,000.
 2. Process according to claim 1, wherein the process is a multi-nozzle electrospinning process comprises steps wherein the high voltage is applied between a spinneret and a collector, or between a separate electrode and a collector, the spinneret comprising a series of spinning nozzles, a stream of a polymer solution comprising a polymer and a solvent is fed to the spinneret, and the polymeric solution exits from the spinneret through the spinning nozzles and transforms under the influence of the high voltage into charged jet streams.
 3. Process according to claim 1, wherein the process is a nozzle-free electrospinning process.
 4. Process according to claim 1, wherein the C/N ratio is in the range of 2-5, and/or the polyamide is a homopolyamide of polyamide 46, polyamide 26, polyamide 4, polyamide 36,
 5. Process according to claim 1, wherein the polyamide is present in an amount of at least 50 wt. % relative to the polymer, and the polyamide has an Mw in the range of 2,000-30,000.
 6. Process according to claim 1, wherein the polymer comprises a second polyamide having a C/N ratio of more than 5.5 and/or an Mw of at more than 35,000, and/or a water soluble polymer.
 7. Process according to claim 1, wherein the solution comprises an additive.
 8. Process according to claim 1, wherein the solvent comprises either (I) water, a water soluble salt, and either methanol, ethanol, glycol and/or glycerin, or (II) formic acid and/or acetic acid, and at least one liquid selected from the group consisting of water, methanol, ethanol, glycol, glycerin, and methyl formate.
 9. Process according to claim 1, comprising one or more of the following steps: a washing step, a drying step, a curing step, and/or a post condensation step.
 10. Nanofibres or nano-web consisting of a polymer composition comprising a polyamide having a C/N ratio of at most 5.5 and an Mw of at most 35,000.
 11. Nanofibres or nano-web consisting of a polymer composition comprising a polyamide having a C/N ratio of less than 5 and a Tm of at least 300° C.
 12. Nanofibres or nano-web consisting of a polymer composition comprising a polyamide having a C/N ratio of more than 5.5 and an Mw of at most 35,000 and a linear or branched polyamide having an Mw of more than 35,000.
 13. Use of nanofibres or a nano-web according to claim 10, or a micro-porous membrane made thereof, for the use of any one of the following applications: molecular separations and filtration, like gas/gas filtration, hot gas filtration, particle filtration, liquid filtration such as micro filtration, ultra filtration, nano filtration, reverse osmosis; waste water purification, oil and fuel filtration electrochemical applications, including electro-dialysis, electro-deionization, batteries and fuel cells; controlled release applications including pharmaceutical and nutraceutical components; pertraction, pervaporation and contactor applications; immobilization of enzymes, and humidifiers, drug delivery; (industrial) wipes, surgical gowns and drapes, wound dressing, tissue engineering, protective clothing, catalyst supports, and coatings.
 14. Separation process wherein nanofibres or a nanoweb according to claim
 10. 